Ground fault detector circuit

ABSTRACT

A ground fault detector providing protection from leakage current to ground by sensing unbalance in supply and neutral conductor currents to actuate a circuit breaker. A solid state circuit including amplifier and detector and having the unbalance signal of a current transformer as an input for energizing a trip coil of the breaker when the unbalance current exceeds a predetermined level. Improved circuitry providing temperature compensation, insensitivity to turn on and other transients and noise, and protection against overheating due to breaker hangup.

United States Patent Boss [54] GROUND FAULT DETECTOR CIRCUIT [72]Inventor: Robert J. Hoes, Plano, Tex.

[73] Assignee: Zlnsco Electrical Products, L/os Angeles, Calif.

[22] Filed: Jan. 10, 1972 [21] Appl. No.: 216,526

52 us.c|.............317/1s n, 317/27 R, 317/33 so, 0. 3 17l& l i3 t3..2l9 51] Int. Cl H02h 3/28 [53 Field of samh....I....317/1s"i5f33, 27 R,33 so, 317/49; 323/60, 70

[56] References Cited UNITED STATES PATENTS 2,957,122 10/ 1960Manteuffel ..323/70 3,512,045 5/1970 Tipton et al. ..3l7/l8 D 51 Oct.24, 1972 Douglas...... Douglas......

.............3l7/l8 D .............3l7/l8 D Primary Examiner-James D.Trammell Attorney-Ford W. Harris, Jr. et a].

[57] ABSTRACT 19 Claims, 4 Drawing Figures Q7551? Z/ men/"0w ,2 POLElama/KEN DETECTOR AMPLIFIER l3 emuurzqacswfl IL TQQNSFGQMERIE i E l3! if'k if i AC/ 1 24s 3 we 9 CR! 20 I 1 RI (R2 1 1 CR4 1 GROUND FAULTDETECTOR CIRCUIT BACKGROUND OF THE INVENTION This invention relates to adetector for ground faults in electrical installations and morespecifically, to a new and improved ground leakage current detector forprotecting personnel by tripping a circuit breaker when a leakagecurrent to ground exceeds a predetermined value. Ground fault detectorshave been in use for some time and typically include some means such asa current transformer for detecting the presence of a ground faultcurrent, and some means for actuating a circuit breaker when the faultcurrent exceeds some preset value. Typical prior art devices are shownin the U.S. Pats. to Schweitzer No. 2,238,570, Adamson No. 2,977,510,and Dalziel No. 3,213,321, and British Pat. No. 446,299.

A present day ground fault detector should be small and inexpensive andreadily adapted for use with small portable electrical equipment as wellas in fixed installations. The detector should be capable of operationto open the main power circuit when a specific small fault currentexists and should operate repeatedly within small tolerances. At thesame time, the detector should be insensitive to temperature variations,noise and other interference and transients due to power turn on.

SUMMARY OF THE INVENTION The ground fault detector is operated with acurrent transformer and a circuit breaker. The current transformersecondary is part of a resonant circuit which provides an unbalancesignal to a tuned, high gain, stable amplifier with ac. and dc.feedback. A detector circuit actuates a solid state switch to trip thebreaker when the amplifier output exceeds a predetermined value. Powerfor the amplifier and detector is supplied by a regulated power supplycapacitively coupled to the power source on the load side of thebreaker. The detector circuit is pulsed in order to prevent damage toany of the circuitry in the event of breaker hangup. The ground faultdetector may be operated with three wire systems and with two wiresystems, and may be operated on one-half cycle of the at; source and oneither half cycle.

One object of this invention is to provide a circuit for detecting thepresence of a current leakage to ground which may be flowing through ahuman body from the hot side of a 110 VAC or 220 VAC power source toground. This may be accomplished by a current transformer which sensesboth the current flowing to the load from a power source and the currentreturning through the neutral. The current transformer is configuredsuch that the current in the primaries, consisting of the supplyconductors and the neutral conductor, subtract with the inducedsecondary current representin g the difference in the current flowing tothe load and that returning on the neutral. If any current path toground is present, the current flowing to the load will be greater thanthat returning through the neutral conductor, and thus a current will beinduced in the secondary winding proportional to the amount of leakagepresent.

Another object of this invention is to provide a means for increasingthe sensitivity of the circuitry to the 60 Hz fault signal whiledecreasing its sensitivity to frequencies above and below thisfrequency. This may be accomplished by tuning the amplifier with aresistor capacitor network.

A third object of this invention is to compensate for the output versustemperature characteristics of the current transformer due to the changein core material initial permeability over the wide temperature range inwhich the ground fault detector must operate. Compensation is achievedby providing a resonant circuit at the secondary winding and choosingthe resonance point of the transformer resonant circuit such that thechange in inductance with temperature moves the resonant peak in amanner which properly shapes the temperature versus outputcharacteristics of the transformer.

Another object of the invention is to provide for pulsing the magnetictrip coil of the breaker such that the duty cycle of the current pulsesto the trip coil is low enough that the coil will not heat sufficientlyto cause insulation breakdown or component failure in the event ofbreaker hang-up and continuation of the fault current.

A further object is to eliminate the possibility of the circuitryactivating the breaker when the breaker is initially set due to settlingand turn on transients in the electronics.

DESCRIPTION OF THE DRAWING FIG. I is a circuit diagram of a ground faultdetector with a three wire system and incorporating one embodiment ofthe invention;

FIG. 2 is a circuit diagram showing an alternative form of thresholddetector for the circuit of FIG. I, which is operable to actuate thecircuit breaker on either the positive or negative going half-cycle ofthe ac. source;

FIG. 3 is a circuit diagram showing an alternative arrangement for thetransformer secondary; and

FIG. 4 is a circuit diagram of another and presently preferredembodiment of the threshold detector with a hold off circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Ground Fault Detector EmployingSingle Polarity Fault Signal Detection In the circuit of FIG. 1, ac.power is provided from a transformer 20 through a circuit breaker 21 andcurrent transformer T1 to the load terminals 4, 5, 6. The ground faultdetector includes a regulated d.c. supply 12, a tuned multistageamplifier 13, a threshold detector l4, and a solid state switch SCRl. Athree wire system is illustrated but the detector is equally suitablefor use with a two wire system and the circuit of FIG. I can beconverted to a two wire system by omitting the supply conductor betweenterminals 2 and 5, and the components associated therewith.

The AC power is supplied at input terminals 1 and 2 and neutral terminal3. AC power passes through the circuit breaker contacts S3 and isdelivered to the load at output terminals 4 and 5. The neutral iscontinuous from input terminal 3 to output temtinal 6.

The fault-sensing device includes current transformer T1 and capacitorC1. The primary windings of T1 consist of the AC load conductors 7 and8, and the neutral conductor 9, along with a circuit test conductor 10.If the sum of the current through conductors 7 and 8 is greater than thecurrent through neutral conductor 9, representing a fault condition, acurrent will be induced in secondary winding 11. Also, if test switch S2is depressed such that a current of a value determined by resistor R1 1flows through conductor 10, a current will be induced in secondarywinding 1 1. Capacitor C1 provides a means for resonating thetransformer secondary 11 such that the sensitivity of the transformer tothe fault current frequency is enhanced while the sensitivity to higherfrequencies is reduced. it also provides a means for temperaturecompensating the change in transformer inductance. The inductance andresistance of secondary 11 and the capacitance of capacitor C1 form aresonant circuit.

The regulated DC power to the solid state circuitry is provided by theregulated supply 12 consisting of capacitors C2, C3 and C4, resistor R1,diodes CR1, CR2, CR3, CR4, zener diode CR5, and transistor Q1. DiodesCR1, CR2, CR3 and CR4 act as a full wave bridge rectifier withconductors 7 and 8 providing the AC power, and neutral conductor 9 actsas the DC ground for the power supply and other circuitry. Capacitors C2and C3 act as current-limiting devices. The reactive impedance of thesecapacitors at the AC power source frequency drops the voltage to thefull wave rectifier to a convenient lower voltage at which thesemiconductor circuitry can operate. Capacitor C4 acts as a filter toremove a majority of the full wave ripple from the bridge rectifier.Zener diode CR provides a stable DC voltage reference for circuitoperation. It is biased through resistor R1. Transistor Q1 is a seriespass transistor which, using CR5 as a reference, regulates the DC outputvoltage under changes in load conditions and aids in further removingpower supply ripple.

The amplifier 13 is composed of transistors Q2, Q3, and Q4, capacitorsC5, C6 and C7, and resistors R2, R3, R4, R5, R6, R7, and R8. The faultor unbalance signal from the secondary winding 11 of transformer Tl ispresented to the emitter of transistor Q2. The amplifier in turnpresents an amplified signal to the threshold detector 14 at thecollector of transistor Q4.

The amplifier l3 incorporates both AC and DC feedback, through resistorsR4 and R5 and capacitor C6, in order to stabilize both the DC operatingpoint and the AC gain over a wide temperature range. The DC operatingpoint at the collector of transistor 04 is essentially at the samepotential as the DC potential established at the junction of biasresistors R2 and R3, less the base to emitter potential of transistor 02and a small drop across resistor R4. This is so since the integratingaction of C6 maintains an essentially constant DC potential at the baseof 02.

AC midfrequency gain of the amplifier 13 is established by resistors R4and R5, which act as a voltage divider to the out-of-phase signal on thecollector of transistor 04. The divided out-of-phase signal is presentedto the base of transistor 02, while the input signal from transfonner T1is presented at the emitter of Q2. The resulting signal amplitudedifferential results in a constant gain over a wide temperature range.

High frequency noise suppression is provided by rolloff capacitor C7.The combination of resistors R7, R8

and capacitor C7 determines the rollolf frequency. Capacitor C5 providesadditional power supply ripple suppression while providing a lowimpedance signal path from the fault-sensing transformer secondary tosignal ground.

The threshold detector 14 consists of transistors 05 and Q6, resistor R9and variable resistor R8. Transistors Q5 and Q6 are connectedcollector-to-base and are normally in the cut-off state. As soon as thepotential from the base to emitter of Q5 exceeds its cutin voltage,current will be supplied to the base of Q6 and it too will begin toconduct, in turn increasing the base to emitter potential of Q5. Thus,regenerative action will occur and both transistors 05 and 06 will berapidly driven into saturation. Current will then flow through resistorR9, presenting a positive voltage to the gate or control element of asolid state switch such as a Triac or a silicon controlled rectifierSCRl.

The threshold voltage, or the base to emitter voltage impressed acrosstransistor 05, is determined by the difference between the DC potentialat the base of Q2 and the DC plus signal potential at the moving arm ofvariable resistor R8. Since the DC potential between the base of Q2 andcollector of Q4 is essentially equivalent, a DC difierential orthreshold level is obtained by the voltage divider action of variableresistor R8. Adjusting the arm of R8 further away from the collector ofQ4 biases the base to emitter junction of Q5. When an AC signal ispresent at the collector of Q4 and the negative going half cycle is ofsufficient amplitude to offset the preset threshold potential, the baseto emitter junction of Q5 will be forward biased and Q5 will conduct.

The problem of actuation of the threshold detector by tum-on transientsduring initial electronic power up (i.e., when the breaker is set), isovercome by connecting the emitter of O5 to capacitor C6, which slowlycharges from a zero potential, and connecting the base of O5 to thecollector resistor of transistor 04, which rapidly obtains a positivepotential at turn-on. This insures that the base-to-emitter junction oftransistor Q5 will remain reverse biased during the turn-on and settlingtime of the electronics.

Tripping of the circuit breaker 21 is accomplished with the switch SCRl,a trip coil Ll, resistor R10 and diodes CR6 and CR7. The trip coil Llmay be a coil of wire wound around a pole piece within the circuitbreaker mechanism thus forming an electromagnet. When a current flowsthrough this coil, a magnetic field is produced which attracts anarmature piece. The motion of this armature towards the pole piecereleases the circuit breaker mechanism, in turn opening breaker contactsS3.

Whenever the threshold detector 14 is activated, a positive potential ispresented at the gate of switch SCRl. This, in turn, causes SCRl toconduct, allowing current to flow from the protected AC power sourceconductors 7 and 8 through diodes CR6 and CR7, resistor R10 and tripcoil L1 to the neutral conductor 9. Resistor R10 limits the current to asafe operating value. Diodes CR6 and CR7 allow current to flow duringthe positive half cycle (with respect to neutral) of either phase of theAC power source.

As soon as the breaker 21 trips, the contact sets S3 open and power tothe trip coil and the electronics is disconnected. In the case of amechanical breaker hang-up, continuous current through the trip coil andSCRl could potentially cause damage due to excessive heating. For thisreason the gate of SCRl is pulsed at a low duty cycle such that itallows only a single half cycle of current to flow through the trip coilper pulse with a long time delay between pulses (i.e., in the order ofone pulse per second). Pulsing of the switch is accomplished byconnecting the emitter of O5 to capacitor C6. As the threshold circuitis activated, transistors 05 and 06 are driven into saturation. Current,therefore, flows from C6 through Q5 and into the base of Q6 which has avery low input impedance due to its saturation state. Capacitor C6 istherefore partially discharged causing the output potential of theamplifier at transistor O4 to immediately rise to power supplypotential. This in turn causes the threshold detector to cut off. Theresult is a short pulse at the gate of the switch SCRl. The detectorwill remain off until the amplifier again stabilizes. This ofiperiod isdetermined by the charging time of capacitor C6, which is primarily afunction of C6 and R4. The pulse width is determined primarily by C6, RSand R9. The charged capacitor C6 provides the driving current for theregenerative feedback circuit so that the detector remains on for only ashort time, and also provides for blocking or maintaining the detectorin a nonoperative condition when the system is initially energized whilethe capacitor is being initially charged, thereby preventing a faultsignal due to turn-on transients.

The phase shift of the amplifier preferably is adjusted such as to causethe breaker contact set to open at a point in the a.c. power cycle whichminimizes contact arcing time. The number of degrees which the faultsignal is shifted with respect to the a.c. power is dependent upon thesum of the phase shifts contributed by the bandpass characteristics ofthe tuned current transformer and by the tuned amplifier. Phase shiftadjustment may be achieved by changing the bandpass characteristics ofthe amplifier by varying the resistance-capacitance time constants inthe feedback network. Desirably the peaks of the a.c. fault signal (andthus the breaker trip point) occur a few degrees before the peaks of thea.c. source supplying trip coil power. Breaker trip at this point in thea.c. power cycle produces contact opening just prior to the zerocrossing point of the a.c. power cycle, minimizing arcing time.Operation of the Circuit Illustrated in FIG. 1

The detector of the invention is primarily intended to protect peoplefrom electrical shock although it also may be utilized to protectequipment from damage due to grounding short circuits.

Under normal circumstances, when no ground fault condition occurs, allof the current leaving the AC power conductors 7 and 8 will returnthrough the neutral conductor 9. The sum of the currents flowing in theprimaries of current transformer T1 will therefore be zero, and nocurrent will be induced in the secondary. When a fault condition occurssuch that a fault current flows from one or both of the AC powerconductors to ground through a person or some other vehicle, the sum ofthe currents in the primaries of T1 will not cancel since a portion ofthe current flowing to the load in conductors 7 or 8 is not returningvia the neutral 9. The resultant current difi'erential will induce acurrent in the secondary of T1 of a magnitude proportional to the faultcurrent. This fault current or unbalance signal is then amplified bytransistors 02, Q3, and 04. If the amplified unbalance signal is ofmagnitude greater than that of the threshold value set at adjustableresistor R8, the threshold detector with transistors 05 and 06 will beactivated. This in turn will present a positive voltage to the gate ofthe switch SCR] which will then conduct, for at least one half cycle,allowing current to flow through trip coil L1. The magnetic fieldproduced by the current flow in L1 will then trip the breaker. Once thebreaker contacts have opened, power to the load and to the electronicsand trip coil L1 will be discontinued.

In order to provide a capability of disabling the electronics for anyreason, On-Ofi Switch S1 is provided. When this switch is opened, nocurrent will flow in resistor R9 when the threshold detector isactivated; and, therefore, the gate potential of SCRl will remain atzero. The switch will never conduct under this condition.

A test switch S2 is provided such that the electronics may befunctionally checked periodically under simulated fault conditions. WhenS2 is depressed, current from AC power conductor 7 will flow throughtransformer Tl, primary conductor 10 and through resistor R11 to ground.R11 is chosen such that the current in primary conductor 10 isequivalent to the fault current required to activate the thresholddetection circuitry. When S2 is depressed, the current flowing inprimary conductor 10 will induce a current in the secondary winding ofT1. This fault test signal will then activate the electronics and tripthe circuit breaker in the same manner as previously described for anormal fault condition.

Ground Fault Detector Employing Double Polarity Fault Signal DetectionFIG. 2 illustrates an alternative form for the threshold detector 14 ofFIG. 1. The threshold detector of FIG. 2 may be inserted into thecircuit of FIG. 1 at terminals 31-40 in lieu of the detector 14 toprovide double phase detection increasing the speed of fault currentdetection. Resistor R8 may be omitted, with the base of transistor 011connected directly to the collector of transistor 04.

The double phase detector of FIG. 2 differs from the threshold detectorshown in FIG. 1 in that it rectifies the 60 Hz AC fault signal and assuch activates as soon as either the positive or negative half cycle ofthe unbalance or fault signal reaches the predetermined threshold level.The detector 14 in FIG. 1 is activated only as soon as the negative halfcycle of the fault signal reaches threshold amplitude. In this way thedouble phase detection can react at least 8 msec. faster than the singlephase detection of the circuit of FIG. 1.

The detector circuit of FIG. 2 consists of a phase splitter 22, afullwave rectifier 23, a Schmitt trigger threshold detection circuit 24,and an SCR driver stage 25. The phase splitter 22 consists of transistorQ11 and resistors R12 and R12. The amplified fault signal is presentedto the base of 011. Resistors R 12 and R12 are chosen such that the ACwaveform at the emitter and collector of transistor Q11 are of equalamplitude and out of phase. These two waveforms are then coupled to therectifier 23 through capacitors C11 and C12. Diodes CR11 through CR14and Resistors R13, R14 and R constitute a full wave rectifier. ResistorR13 forms a voltage divider with R14 and R15 such that all diodes areconducting when no AC signal is applied. The DC potential at the base oftransistor 012 is adjusted with variable resistor R15. When an AC signalis present at the base of Q1 1, it will be presented to the diode bridgeas a signal of equal amplitude but of opposite polarity. The positivegoing signal will cut its respective diodes off while the negative goingsignal will increase conduction of its respective diodes. This actionresults in a full wave rectified, positive AC fault signal riding on anadjustable DC level.

The rectified unbalance signal is then presented to the thresholddetection circuit (Schmitt trigger) 24, consisting of transistors Q12and Q13, resistors R16, R17, R18, R19 and R20. When the signal at thebase of Q12 is below threshold value, Q12 is cut off and Q13 is in itsnormal active region. Resistors R16, R18 and R19 fonn a voltage dividerwhich determine the base potential of Q13 and as such the emitterpotential of Q13, which is equivalent to the emitter potential of Q12,establishes the threshold potential at which the circuit will operate.When the signal potential at the base of 012 increases beyond thepotential of the emitter plus a small cut-in voltage, Q12 will conductlowering the base potential of Q13. Transistor Q13 will cut off and thelowering of its emitter potential will drive Q12 into saturation. Theregenerative effect insures a rapid response to a signal whichis greaterthan the preset threshold level. The threshold level is adjusted byincreasing or decreasing the resistance of variable resistor R15. Thisraises or lowers the DC potential upon which the rectified AC signal isriding.

The SCR driver stage 25 consists of transistors Q14 and Q15 andresistors R21, R22, R23 and R24. Transistor Q14 is an invertingamplifier whose gain is determined by the ratio of R22 and R23. When theSchmitt trigger is in its stable state (untriggered), the potential atthe collector of Q13 drives the potential at the collector of Q14 a fewvolts above the voltage at the emitter of Q15. This holds 015 in cut-offand thus maintains the gate potential of the switch SCR] at zero. Whenthe Schmitt trigger is tripped, the collector voltage of 013 increasesto a point as determined by the voltage divider R and R21. This drivesthe collector potential of Q14 a few volts below the potential of theemitter of 015 (not to zero volts). Transistor Q15 then conductsdischarging the large feedback capacitor in the amplifier. The resultantcurrent through R24 increases the SCR gate potential and causes switchconduction. This in turn causes the circuit breaker to trip aspreviously explained in conjunction with FIG. 1.

Since the current to drive the SCR control gate comes from a capacitordischarging, it will only last for a short period of time as determinedby the capacitor C6 and resistor R24. This action results in a shortpulse of current flowing in the SCR]. This provision is made in case ofa mechanical breaker hand-up whereby continuous current would flowthrough the trip coil causing excessive heating. The time durationbetween pulses is dependent on the resetting time of the amplifier,which is a second or so.

The SCR driver stage 25 also eliminates the possibility of SCR trippingduring initial power-up due to turnon transients. At initial turn-on thepotential of the feedback capacitor C6 in the amplifier 13, and thus theemitter of transistor Q14 remains very low during the turn on transientperiod. Although Schmitt trigger tripping may occur during this period,the voltage at the base of 015 will never go below the emitter potentialof Q15 and thus Q15 will remain cut off.

Operation of the Circuit Illustrated in FIG. 2

When a fault current is present, because of a person being shocked orsome other reason, it will be sensed by the current transformer I1 andamplified as explained in conjunction with FIG. 1. The amplified ACsignal will then be presented to the input of the double phase detectioncircuit of FIG. 2. The signal will be full wave rectified by the actionof the phase splitter 22 and the bridge rectifier 23. The full waverectified signal will appear at the base of Schmitt trigger transistorQ12, riding on a DC potential which is adjustable with R15. If thepotential of the AC signal positive peaks plus the adjusted DC level onwhich it is riding is greater than the threshold level established bythe resistive divider within the Schmitt trigger, the Schmitt triggerwill switch to its on state. The voltage excursion of the collector ofQ13 is amplified by 014 and in turn sends the transistor 015 intoconduction. When this occurs, a positive potential will appear at thegate of SCRI driving it into conduction, in turn activating the circuitbreaker trip mechanism.

Alternative Embodiment of Ground Fault Detector Employing DoublePolarity Fault Signal Detection FIGS. 3 and 4 illustrate another andpresently preferred embodiment of a ground fault interrupter utilizingdouble polarity fault signal detection. The filter circuit 45 may beinserted between the capacitor C1 and transistor 02 of FIG. 1. Thecircuit of FIG. 4 may be inserted into the circuit of FIG. 1 atterminals 31-40 in the same manner as the circuit of FIG. 2. The circuitof FIG. 4 includes the phase splitter 22, the full wave rectifier 23, ahold off circuit 46, and a threshold detector and SCR pulsing circuit47.

The circuit of FIGS. 3 and 4 difi'ers from the circuits in FIGS. 1 and 2in that it is designed for greater noise immunity and for betteroperation under high fault current conditions. In essence, this circuitcombines the amplifier and regenerative feedback circuit illustrated inFIG. 1 with the phase splitter circuitry in FIG. 2 in order to achieve adouble polarity fault detection circuit with less transistors andimproved operation characteristics. Components corresponding to those ofFIGS. 1 and 2 are identified by the same reference numerals.

In the filter circuit 45 (FIG. 3), capacitor C30 and resistor R30provide a means for resonating the secondary 11 of transformer T1 suchthat the sensitivity of the transformer to the fault current frequencyis enhanced while the sensitivity to higher frequencies is reduced. Theyalso provide a means for temperature compensating the change intransformer inductance. Capacitor C30 determines the resonant frequencyand resistor R30 determines the response time of the torrid transformerT1. Capacitor Cl is added for additional filtering of high currentswitching transients. Diodes CR36 and CR3! provide high voltagetransient protection to the input of the amplifier.

The threshold detector 47 includes transistors Q36 and Q37, resistorsR34, R35, R36 and diodes CR 32 and CR33. Transistors Q36 and Q37 areconnected to collector-to-base and are normally in the cut-off state. Assoon as the potential from the base to emitter of Q36 exceeds its cut-involtage, current will be supplied to the base of Q37; and it too willbegin to conduct, in turn increasing the base to emitter potential ofQ36. Thus, regenerative action will occur and both transistors Q36 and037 will be rapidly driven into saturation. Current will then flowthrough resistor R24, presenting a positive voltage to the gate of SCRl.

The threshold voltage, or the base-to-emitter voltage impressed acrosstransistor Q36, is determined by the difference between the DC potentialat the base of Q36 and the DC plus signal potential at the junction ofthe full wave rectifier and the variable resistor R15. The DC potentialat the base of Q36 is determined by the divider action of R35 and R36and by the voltage drop across temperature compensation diodes CR32 andCR33. Adjusting the armature of resistor R15 biases the base-to-emitterjunction of Q36 below threshold voltage. When an AC signal is present atthe output of the rectifier and the positive going half cycle is ofsufficient amplitude to offset the preset threshold potential, thebase-to-emitter junction of Q36 will be forward biased and Q36 willconduct. Capacitors C31 and C33 provide filtering for the prevention ofnuisance tripping from transient pick-up within the circuitry.

The hold-off circuit 46 prevents actuation of the threshold detector dueto turn-on transients during initial electronics power up, i.e., whenthe breaker is set. The emitter of 036 is connected to capacitor C32,through diode CR31. Capacitor C32 slowly charges through resistor R33from a zero potential at power up, insuring that the base-to-emitterjunction of Q36 will remain reverse biased during the turn-on andsettling time of the electronics.

The hold-off circuit 46 differs from that in FIGS. 1 and 2 in that it isseparate from the amplifier. This is done so that the hold-off andtrigger operation of the threshold detector is not affected by theoperation of the amplifier which can become saturated by large faultcurrents and thus cause a threshold detector malfunction. In theconfiguration of FIG. 4 threshold and SCR pulsing action is maintainedregardless of fault current magnitude.

Pulsing of the switch SCRl is accomplished by connection of the emitterof 036 to capacitors C11 and C12 through the rectifier. As the thresholdcircuit is activated, Q36 and 037 are driven into saturation. Current,therefore, flows from C11 and C12 through Q36 and into the base of Q37which has a very low input impedance due to its saturation state.Capacitors C11 and C12 are thus discharged. This in turn causes thethreshold detection circuitry to cut off. The result is a short pulse atthe gate of SCRl. The threshold circuitry will remain off until C11 andC12 fully charge. This off period is determined by the charging time ofC11 and C12 through R13. The pulse width is determined primarily by C11,C12, R24 and the gate impedance of SCR].

The operation of the circuit of FIGS. 3 and 4 corresponds to that of thecircuits of FIGS. 1 and 2.

lclaim:

1. in a ground fault detector for operation with a circuit breaker, acurrent transformer, and an a.c. power source with at least one supplyconductor and a neutral or return conductor,

with the supply conductor connected through a breaker contact set to atransformer primary winding and with the neutral conductor connected toanother transformer primary winding, and with the transformer secondarywinding providing an unbalance signal when there is an unbalance in thecurrents in the supply conductor and the neutral conductor,

the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to saidamplifier input for amplifying said unbalance signal;

detector means having said amplifier output as an input for detectingwhen said amplifier output exceeds a predetermined value and producing acontrol signal; trip means having said control signal for an input foractuating said circuit breaker and opening said Contact set;

a dc. power supply for said amplifier means and detector means; i

said detector means including a first transistor switched intoconduction when said amplifier output exceeds said predetermined value,and

said amplifier means including a first capacitor charged from said d.c.supply through a first resistor; and

circuit means interconnecting said first capacitor and first transistorfor discharging said first capacitor through the conducting firsttransistor, with the discharged capacitor reducing the amplifier outputto cut off said first transistor and provide a control signal pulse,

with the pulse duration being a function of the capacitor discharge timeand with the interpulse interval being a function of the capacitorcharge time.

2. A ground fault detector as defined in claim 1 wherein said circuitmeans for connecting said transformer includes a second capacitorconnected across said secondary winding forming a resonant circuit, saidsecond capacitor having a capacitance to resonate with said secondarywinding at a frequency in the order of that of the a.c. source, with thechange of inductance of the secondary winding with change in temperatureshifting the resonant frequency of the resonant circuit maintaining thesecondary winding output substantially constant as the initialpermeability of the transformer core material changes with change intemperature.

3. A ground fault detector as defined in claim 2 wherein said amplifiermeans includes a multistage negative feedback amplifier having a secondtransistor in one stage and a third transistor in a succeeding stage,with the signal to be amplified connected through the collector andemitter of said second transistor to the next stage, and

means defining an ac. feedback path from the output of said thirdtransistor to the base of said second transistor providing a signalout-of-phase with the signal to be amplified, and

l 1 means defining a dc. feedback path comprising a third capacitorconnected between said second transistor base and circuit groundproviding a substantially constant d.c. potential at said secondtransistor base.

4. A ground fault detector as defined in claim 3 including means forreverse biasing said first transistor during power build-up after saidpower supply is energized and having a resistive circuit in an outputstage of said amplifier connected to the base of said first transistor,with build-up at said base preceding build-up at said first capacitor.

5. A ground fault detector as defined in claim 3 including a thirdcapacitor connecting the input of said dc. power supply to the a.c.power source conductor on the load side of the circuit breaker.

6. A ground fault detector as defined in claim 3 wherein the circuitbreaker has a magnetic trip mechanism with a magnetic armature, and saidtrip means includes a coil of wire wound around said armature and asolid state switch connected to the a.c. power source in series withsaid coil, with said switch actuated by said control signal of saiddetector means.

7. In a ground fault detector for operation with a circuit breaker, acurrent transformer, and an a.c. power source with at least one supplyconductor and a neutral or return conductor,

with the supply conductor connected through a breaker contact set to atransformer primary winding and with the neutral conductor connected toanother transformer primary winding, and with the transformer secondarywinding providing an unbalance signal when there is an unbalance in thecurrents in the supply conductor and the neutral conductor,

the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to saidamplifier input for amplifying said unbalance signal,

including a first capacitor connected across said secondary windingforming a resonant circuit, said capacitor having a capacitance toresonate with said secondary winding at a frequency in the order of thatof the a.c. source, with the change of inductance of the secondarywinding with change in temperature shifting the resonant frequency ofthe resonant circuit maintaining the secondary winding outputsubstantially constant as the initial permeability of the transformercore material changes with change in temperature;

detector means having said amplifier output as an input for detectingwhen said amplifier output exceeds a predetermined value and producing acontrol signal; and

trip means having said control signal for an input for actuating saidcircuit breaker and opening said contact set.

8. A ground fault detector as defined in claim 7 wherein said amplifiermeans includes a multistage negative feedback amplifier having a firsttransistor in one stage and a second transistor in a succeeding stage,with the signal to be amplified connected through the collector andemitter of said first transistor to the next stage, and

means defining an a.c. feedback path from the output of said secondtransistor to the base of said first transistor providing a signalout-of-phase with the signal to be amplified, and

means defining a dc. feedback path comprising a second capacitorconnected between said first transistor base and circuit groundproviding a substantially constant d.c. potential at said firsttransistor base.

9. In a ground fault detector for operation with a circuit breaker, acurrent transformer, and an a.c. power source with at least one supplyconductor and a neutral or return conductor,

with the supply conductor connected through a breaker contact set to atransformer primary winding and with the neutral conductor connected toanother transformer primary winding, and with the transformer secondarywinding providing an unbalance signal when there is an unbalance in thecurrents in the supply conductor and the neutral conductor,

the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to saidamplifier input for amplifying said unbalance signal;

detector means having said amplifier output as an input for detectingwhen said amplifier output exceeds a predetermined value and producing acontrol signal; and

trip means having said control signal for an input for actuating saidcircuit breaker and opening said contact set;

said amplifier means including a multistage negative feedback amplifierhaving a first transistor in one stage and a second transistor in asucceeding stage, with the signal to be amplified connected through thecollector and emitter of said first transistor to the next stage, and

means defining an a.c. feedback path from the output of said secondtransistor to the base of said first transistor providing a signalout-of-phase with the signal to be amplified, and

means defining a dc. feedback path comprising a first capacitorconnected between said first transistor base and circuit groundproviding a substantially constant d.c. potential at said firsttransistor base.

10. A ground fault detector as defined in claim 9 wherein said detectormeans includes a third transistor switched into conduction when saidamplifier output exceeds said predetermined value, and

said amplifier means includes a second capacitor charged from said d.c.supply through a first resistor; and

circuit means interconnecting said second capacitor and third transistorfor discharging said second capacitor through the conducting thirdtransistor, with the discharged capacitor reducing the amplifier outputto cut off said third transistor and provide a control signal pulse,

with the pulse duration being a function of the capacitor discharge timeand with the interpulse interval being a function of the capacitorcharge time.

11. In a ground fault detector circuit for operation with a circuitbreaker, a current transformer, and an a.c. power source with at leastone supply conductor and a neutral or return conductor,

with the supply conductor connected through a breaker contact set to atransformer primary winding and with the neutral conductor connected toanother transformer primary winding, and with the transformer secondarywinding providing an unbalance signal when there is an unbalance in thecurrents in the supply conductor and the neutral conductor,

the improvement comprising in combination:

amplifier means having an input and an output;

circuit means for connecting said transformer secondary to saidamplifier input for amplifying said unbalance signal;

detector means having said amplifier output as an input for detectingwhen said amplifier output exceeds a predetermined value and producing acontrol signal;

trip means having said control signal for an input for actuating saidcircuit breaker and opening said contact set;

means for desensitizing the circuit to unwanted transients which may bedetected as an unbalance signal;

means for pulsing the trip means to limit heat generation in the breakercoil and associated electronics; and

means for suppressing electronics turn-on transients which may bedetected as an unbalance signal.

12. A ground fault detector circuit as defined in claim 11 wherein saidmeans for suppressing includes a resistor-capacitor network for tuningsaid amplifier means to have a passband at the frequency of the ac.power source.

13. A ground fault detector circuit as defined in claim 11 wherein:

said trip means includes a solid state switch having a control element;

said detector means includes a DC level adjustment and a regenerativefeedback circuit for driving said switch control element;

with said DC level adjustment located at the output of said amplifiermeans so that the AC signal at the amplifier output is impressed on avariable DC potential; and

capacitor providing the driving current for said feedback circuit; and

with said capacitor comprising said means for suppressing transients byblocking said detector means operation during initial charging of said15? g r iind fault detector circuit as defined in claim 11 wherein:

said detector means includes a phase emitter, a diode rectifier, a DClevel adjustment, and a threshold sensing circuit;

with said phase splitter coupled to said diode rectifier such that theAC signal at the input of the phase splitter is full wave rectified atthe output of the diode rectifier;

with said DC level adjustment connected to the output of the dioderectifier such that the full wave rectified AC signal is impressed on avariable DC voltage;

with said threshold sensing device comprising a Schmitt trigger orunijunction or other bistable or mono-stable, circuit which is activatedwhen the sum of the DC potential and rectified AC potential reaches saidpredetermined value.

16. A ground fault detector circuit as defined in claim 11 includingmeans for adjusting the phase shift of said amplifier means such thatthe peaks of the control signal occur a few degrees before the peaks ofthe AC power cycle supplying the trip means power for causing thecircuit breaker contact set to open just prior to the zero crossingpoint of the AC power cycle minimizing contact arcing time.

17. A ground fault detector as defined in claim 11 wherein said meansfor desensitizing comprises a filter network of first and secondcapacitors and a resistor, with said first capacitor connected acrossthe transformer secondary winding, and with one terminal of said secondcapacitor connected to one terminal of the secondary winding and theother terminal of said second capacitor connected to said resistor whichis connected to the other terminal of the secondary winding, wherebysaid second capacitor and resistor and the transformer inductancedetermine the response frequency and bandwidth of the filter network,with the output of said filter network appearing at the tenninals ofsaid second capacitor.

18. A ground fault detector as defined in claim 1 wherein said circuitmeans interconnecting said first capacitor and first transistor includesa rectifier with its with said regenerative feedback circuit in acut-off u put c nn t a first transistor. and a phase state until the sumof said AC signal and said DC potential upon which the AC signal isimpressed is greater than said predetermined value, at which time saidregenerative feedback circuit will conduct providing current to saidcontrol element.

14. A ground fault detector circuit as defined in claim 13 including adc. power source, and

wherein said regenerative feedback circuit includes a capacitor chargedfrom said d.c. source and discharged into said detector means, with saidsplitter driven by the amplifier output and connected to said rectifieras an input.

19. A ground fault detector as defined in claim 18 including anadditional capacitor connected between the input of said detector meansand circuit ground and connected to said dc. power supply through anadditional resistor for charging when the detector is energized forholding said detector means in an off-state for a few seconds whiletransients subside.

1. In a ground fault detector for operation with a circuit breaker, acurrent transformer, and an a.c. power source with at least one supplyconductor and a neutral or return conductor, with the supply conductorconnected through a breaker contact set to a transformer primary windingand with the neutral conductor connected to another transformer primarywinding, and with the transformer secondary winding providing anunbalance signal when there is an unbalance in the currents in thesupply conductor and the neutral conductor, the improvement comprisingin combination: amplifier means having an input and an output; circuitmeans for connecting said transformer secondary to said amplifier inputfor amplifying said unbalance signal; detector means having saidamplifier output as an input for detecting when said amplifier outputexceeds a predetermined value and producing a control signal; trip meanshaving said control signal for an input for actuating said circuitbreaker and opening said contact set; a d.c. power supply for saidamplifier means and detector means; said detector means including afirst transistor switched into conduction when said amplifier outputexceeds said predetermined value, and said amplifier means including afirst capacitor charged from said d.c. supply through a first resistor;and circuit means interconnecting said first capacitor and firsttransistor for discharging said first capacitor through the conductingfirst transistor, with the discharged capacitor reducing the amplifieroutput to cut off said first transistor and provide a control signalpulse, with the pulse duration being a function of the capacitordischarge time and with the interpulse interval being a function of thecapacitor charge time.
 2. A ground fault detector as defined in claim 1wherein said circuit means for connecting said transformer includes asecond capacitor connected across said secondary winding forming aresonant circuit, said second capacitor having a capacitance to resonatewith said secondary winding at a frequency in the order of that of thea.c. source, with the change of inductance of the secondary winding withchange in temperature shifting the resonant frequency of the resonantcircuit maintaining the secondary winding output substantially constantas the initial permeability of the transformer core material changeswith change in temperature.
 3. A ground fault detector as defined inclaim 2 wherein said amplifier means includes a multistage negativefeedback amplifier having a second transistor in one stage and a thirdtransistor in a succeeding stage, with the signal to be amplifiedconnected through the collector and emitter of said second transistor tothe next stage, and means defining an a.c. feedback path from the outputof said third transistor to the base of said second transistor providinga signal out-of-phase with the signal to be amplified, and meansdefining a d.c. feedback path comprising a third capacitor connectedbetween said second transistor base and circuit ground providing asubstantially constant d.c. potential at said second transistor base. 4.A ground fault detector as defined in claim 3 including means forreverse biasing said first transistor during power build-up after saidpower supply is energized and having a resistive circuit in an outputstage of said amplifier connected to the base of said first transistor,with build-up at said base preceding build-up at said first capacitor.5. A ground fault detector as defined in claim 3 including a thirdcapacitor connecting the input of said d.c. power supply to the a.c.power source conductor on the load side of the circuit breaker.
 6. Aground fault detector as defined in claim 3 wherein the circuit breakerhas a magnetic trip mechanism with a magnetic armature, and said tripmeans includes a coil of wire wound around said armature and a solidstate switch connected to the a.c. power source in series with saidcoil, with said switch actuated by said control signal of said detectormeans.
 7. In a ground fault detector for operation with a circuitbreaker, a current transformer, and an a.c. power source with at leastone supply conductor and a neutral or return conductor, with the supplyconductor connected through a breaker contact set to a transformerprimary winding and with the neutral conductor connected to anothertransformer primary winding, and with the transformer secondary windingproviding an unbalance signal when there is an unbalance in the currentsin the supply conductor and the neutral conductor, the improvementcomprising in combination: amplifier means having an input and anoutput; circuit means for connecting said transformer secondary to saidamplifier input for amplifying said unbalance signal, including a firstcapacitor connected across said secondary winding forming a resonantcircuit, said capacitor having a capacitance to resonate with saidsecondary winding at a frequency in the order of that of the a.c.source, with the change of inductance of the secondary winding withchange in temperature shifting the resonant frequency of the resonantcircuit maintaining the secondary winding output substantially constantas the initial permeability of the transformer core material changeswith change in temperature; detector means having said amplifier outputas an input for detecting when said amplifier output exceeds apredetermined value and producing a control signal; and trip meanshaving said control signal for an input for actuating said circuitbreaker and opening said contact set.
 8. A ground fault detector asdefined in claim 7 wherein said amplifier means includes a multistagenegative feedback amplifier having a first transistor in one stage and asecond transistor in a succeeding stage, with the signal to be amplifiedconnected through the collector and emitter of said first transistor tothe next stage, and means defining an a.c. feedback path from the outputof said second transistor to the base of said first transistor providinga signal out-of-phase with the signal to be amplified, and meansdefining a d.c. feedback path comprising a second capacitor connectedbetween said first transistor base and circuit ground providing asubstantially constant d.c. potential at said first transistor base. 9.In a ground fault detector for operation with a circuit breaker, acurrent transformer, and an a.c. power source with at least one supplyconductor and a neutral or return conductor, with the supply conductorconnected through a breaker contact set to a transformer primary windingand with the neutral conductor connected to another transformer primarywinding, and with the transformer secondary winding providing anunbalance signal when there is an unbalance in the currents in thesupply conductor and the neutral conductor, the improvement comprisingin combination: amplifier means having an input and an output; circuitmeans for connecting said transformer secondary to said amplifier inputfor amplifying said unbalance signal; detector means having saidamplifier output as an input for detecting when said amplifier outputexceeds a predetermined value and producing a control signal; and tripmeans having said control signal for an input for actuating said circuitbreaker and opening said contact set; said amplifier means including amultistage negative feedback amplifier having a first transistor in onestage and a second transistor in a succeeding stage, with the signal tobe amplified connected through the collector and emitter of said firsttransistor to the next stage, and means defining an a.c. feedback pathfrom the output of said second transistor to the base of said firsttransistor providing a signal out-of-phase with the signal to beamplified, and means defining a d.c. feedback path comprising a firstcapacitor connected between said first transistor base and cIrcuitground providing a substantially constant d.c. potential at said firsttransistor base.
 10. A ground fault detector as defined in claim 9wherein said detector means includes a third transistor switched intoconduction when said amplifier output exceeds said predetermined value,and said amplifier means includes a second capacitor charged from saidd.c. supply through a first resistor; and circuit means interconnectingsaid second capacitor and third transistor for discharging said secondcapacitor through the conducting third transistor, with the dischargedcapacitor reducing the amplifier output to cut off said third transistorand provide a control signal pulse, with the pulse duration being afunction of the capacitor discharge time and with the interpulseinterval being a function of the capacitor charge time.
 11. In a groundfault detector circuit for operation with a circuit breaker, a currenttransformer, and an a.c. power source with at least one supply conductorand a neutral or return conductor, with the supply conductor connectedthrough a breaker contact set to a transformer primary winding and withthe neutral conductor connected to another transformer primary winding,and with the transformer secondary winding providing an unbalance signalwhen there is an unbalance in the currents in the supply conductor andthe neutral conductor, the improvement comprising in combination:amplifier means having an input and an output; circuit means forconnecting said transformer secondary to said amplifier input foramplifying said unbalance signal; detector means having said amplifieroutput as an input for detecting when said amplifier output exceeds apredetermined value and producing a control signal; trip means havingsaid control signal for an input for actuating said circuit breaker andopening said contact set; means for desensitizing the circuit tounwanted transients which may be detected as an unbalance signal; meansfor pulsing the trip means to limit heat generation in the breaker coiland associated electronics; and means for suppressing electronicsturn-on transients which may be detected as an unbalance signal.
 12. Aground fault detector circuit as defined in claim 11 wherein said meansfor suppressing includes a resistor-capacitor network for tuning saidamplifier means to have a passband at the frequency of the a.c. powersource.
 13. A ground fault detector circuit as defined in claim 11wherein: said trip means includes a solid state switch having a controlelement; said detector means includes a DC level adjustment and aregenerative feedback circuit for driving said switch control element;with said DC level adjustment located at the output of said amplifiermeans so that the AC signal at the amplifier output is impressed on avariable DC potential; and with said regenerative feedback circuit in acut-off state until the sum of said AC signal and said DC potential uponwhich the AC signal is impressed is greater than said predeterminedvalue, at which time said regenerative feedback circuit will conductproviding current to said control element.
 14. A ground fault detectorcircuit as defined in claim 13 including a d.c. power source, andwherein said regenerative feedback circuit includes a capacitor chargedfrom said d.c. source and discharged into said detector means, with saidcapacitor providing the driving current for said feedback circuit; andwith said capacitor comprising said means for suppressing transients byblocking said detector means operation during initial charging of saidcapacitor.
 15. A ground fault detector circuit as defined in claim 11wherein: said detector means includes a phase emitter, a dioderectifier, a DC level adjustment, and a threshold sensing circuit; withsaid phase splitter coupled to said diode rectifier such that the ACsignal at the input of the phase splitteR is full wave rectified at theoutput of the diode rectifier; with said DC level adjustment connectedto the output of the diode rectifier such that the full wave rectifiedAC signal is impressed on a variable DC voltage; with said thresholdsensing device comprising a Schmitt trigger or unijunction or otherbistable or mono-stable, circuit which is activated when the sum of theDC potential and rectified AC potential reaches said predeterminedvalue.
 16. A ground fault detector circuit as defined in claim 11including means for adjusting the phase shift of said amplifier meanssuch that the peaks of the control signal occur a few degrees before thepeaks of the AC power cycle supplying the trip means power for causingthe circuit breaker contact set to open just prior to the zero crossingpoint of the AC power cycle minimizing contact arcing time.
 17. A groundfault detector as defined in claim 11 wherein said means fordesensitizing comprises a filter network of first and second capacitorsand a resistor, with said first capacitor connected across thetransformer secondary winding, and with one terminal of said secondcapacitor connected to one terminal of the secondary winding and theother terminal of said second capacitor connected to said resistor whichis connected to the other terminal of the secondary winding, wherebysaid second capacitor and resistor and the transformer inductancedetermine the response frequency and bandwidth of the filter network,with the output of said filter network appearing at the terminals ofsaid second capacitor.
 18. A ground fault detector as defined in claim 1wherein said circuit means interconnecting said first capacitor andfirst transistor includes a rectifier with its output connected to saidfirst transistor, and a phase splitter driven by the amplifier outputand connected to said rectifier as an input.
 19. A ground fault detectoras defined in claim 18 including an additional capacitor connectedbetween the input of said detector means and circuit ground andconnected to said d.c. power supply through an additional resistor forcharging when the detector is energized for holding said detector meansin an off-state for a few seconds while transients subside.